Diffraction creates a limit on imaging resolution. This limit is sometimes called the diffraction barrier. The diffraction barrier for a given imaging system is determined by the optics of the imaging system.
Spatial Resolution: For a conventional imaging system with a numerical aperture NA, two point sources of equal intensity light can be spatially resolved by the system only if the distance between the centers of two spots of light (formed in the image plane by light from the two point sources) is equal to at least the Abbe X-Y Resolution. As used herein, “Abbe X-Y Resolution” means λ/2NA, where NA is numerical aperture of the imaging system, and λ is wavelength. Thus, for any given imaging system with a given numerical aperture, Abbe X-Y Resolution is a limit, imposed by the physical laws of diffraction, on the system's spatial resolution that can be achieved by conventional optics.